A comparison of the density of NADPH-diaphorase-reactive neurons in the fascia dentata and Ammon's horn between 6-month and 12-month old Dark Agouti rats

The present study aimed to assess the developmental progress of the hippocampal nitric oxide (NO) system within adulthood by comparing the density of NO-producing neurons in the fascia dentata and Ammon's horn in two groups of adult male rats using NADPH-diaphorase (NADPH-d) histochemistry. One group comprised 6-month-old rats (early adulthood), and the other 12- month-old rats (middle-adulthood). Areal density (number of neurons per unit area) of NADPH-d positive neurons along the three hippocampal axes (septo- temporal, transverse and radial axes) were subjected to quantitative analyses. There were significant variations in the density of NADPH-d- reactive neurons along the transverse and radial axes of the hippocampus, similar to what have been described previously. Comparison between 6-month and 12-month-old rats indicated a substantial reduction in the density of NADPH-d-reactive neurons in the fascia dentata (69%) and Ammon's horn (54%) of the latter group. This reduction was relatively uniform along the septotemporal and radial axes, but appeared to be more pronounced in the fascia dentata and in the proximal region of Ammon's horn. Our finding showed that the hippocampal NO system can undergo significant changes within adulthood. It further highlighted the possibility that an age-related reduction in the capacity to produce NO may not be directly responsible for the cognitive decline associated with senescence, but rather predisposes neuronal degeneration in later life.